The present invention relates to a liquid crystal display element and, more particularly, to a polymer dispersion type liquid crystal display element in which liquid crystals are dispersed in polymer. The polymer dispersion type liquid crystal according to the present invention includes both of polymer dispersion type liquid crystal in a narrow sense in which liquid crystal droplets are dispersed and held in a continuous phase in a polymer matrix and the so-called polymer network liquid crystal in which liquid crystal droplets are dispersed and held in networks of polymer matrix in the form of a three dimensional network.
The liquid crystal display element, which is a display element featuring low profile, lightweight and low power consumption, has been widely used as a display screen of word processors and of TVs hitherto. Of the known liquid crystal display elements, the polymer dispersion type liquid crystal display element using light-scattering mode, which requires no polarizers and also requires no alignment layer treatment to substrates, enables a simplified structure and bright and good contrast ratio display. In particular, when the polymer dispersion type liquid crystal display element is applied to projection type liquid crystal display adapted to project images on the screen, a large image of bright and excellent contrast ratio can be easily created on the screen, and accordingly the use of the polymer dispersion type liquid crystal display element in this field is being progressing.
The polymer dispersion type display element has however a delay in development, as compared with the liquid crystal display elements of TN (Twisted Nematic) mode and STN (Super Twisted Nematic) mode and still has the following disadvantages. Since the polymer dispersion type liquid crystal is such that microscopic liquid crystal droplets of micron order are confined in the polymer matrix, liquid crystal molecules in the liquid crystal droplets are affected by physical restrictive force (hereinafter it is called as xe2x80x9canchoringxe2x80x9d) from an interfacial boundary of the polymer matrix. Because of this, the polymer dispersion type liquid crystal display element is poorer in response of the liquid crystal molecules to electric field than other types of liquid crystal display elements and has a hysteresis that creates a difference in transmittance of the element between at a raised voltage and at a dropped voltage. Further, since the anchoring strength varies depending on temperature of the element, as ambient temperature around the element varies, the transmittance characteristics of the element relevant to the response to electric field and to driving voltage vary considerably. Due to this, although the polymer dispersion type liquid crystal display element holds promise as the coming generation liquid crystal display element, the element of high reliability with satisfactory performance have not yet been realized in the present circumstances.
Following techniques for the polymer dispersion type liquid crystal display element have been hitherto disclosed.
(1) Disclosed by Flat Panel Display ""91, on page 221, published by NIKKEI BP and others is the technique according to which after a compatible mixture of a liquid crystal material and polymerizable monomer is injected in between two opposing substrates, the compatible mixture is irradiated with ultraviolet from above of the substrates under a given temperature condition, to polymerize the monomer while phase separation of the liquid crystal is produced, to thereby produce the polymer dispersion type liquid crystal in which liquid crystals are dispersed in polymer matrix or are dispersed with continuously linked to each other.
(2) Disclosed by Japanese Laid-open Patent Publication No. Hei 5(1993)-158020 is the technique of controlling phase separation by concentration of polymerization initiator in the liquid-crystal-polymer mixture, polymerization temperature and intensity of ultraviolet being all controlled simultaneously.
(3) Disclosed by Japanese Laid-open Patent Publication No. Hei 5(1993)-224180 is the technique of controlling a rate of polymerization of monomers in the guest host type of polymer dispersion type liquid crystal display element.
(4) Disclosed by Japanese Laid-open Patent Publication No. Hei 5(1993)-158020 is the technique of improving intensity of ultraviolet from a conventional range of about 10 mW/cm2 (cf. Symposium on page 414 of The 21st Liquid Crystal Symposium by Mr. Fujikake and others, for example) to the range of from 0.5 mW/cm2 or more to 100 mW/cm2 or less.
(5) Disclosed by Japanese Laid-open Patent Publication No. Hei 5(1993)-127174 is the technique according to which intensity of ultraviolet is set to be 15 mW/cm2 or more when a radical polymerization initiator is used, while on the other hand, the intensity of ultraviolet is set to be in the range of from 100 mW/cm2 or more to 150 mW/cm2 or less when an ionic polymerization initiator is used.
(6) Disclosed by Japanese Laid-open Patent Publication No. Hei 6(1994)-194629 is the technique on a surface temperature of a liquid crystal panel irradiated with ultraviolet, according to which polymerization is produced under temperatures higher than thermal phase separation temperature by a minimum requiring extent, to allow for solubility limit of liquid crystals.
However, these conventional techniques were not enough to solve the abovesaid problems satisfactorily and were also disadvantageous in that it takes much time to accomplish the phase separation by, for example, irradiation of ultraviolet (it takes much time to solidify polymer matrix), due to which great variations in size of liquid crystal droplets and interval between neighboring liquid crystal droplets are caused. Also, the conventional techniques involve the problem that since the anchoring strength of interface liquid crystal/polymer is not adequately adjusted, the response to electric field is not sufficient and the optical hysteresis in high temperature range is as large as 3 to 5% and also the optical hysteresis in low temperature range (less than 10xc2x0 C.) increases further.
At present, what is physical value that controls the optical hysteresis directly is not thoroughly clarified. For this reason, the measurements to improve the optical hysteresis effectively have not yet been found out.
The present invention as a group has been made in the light of the present circumstances described above. It is the primary object of the present invention to develop a new technique for determining optical hysteresis precisely so that a polymer dispersion type liquid crystal display element of improved optical hysteresis can be provided by the application of the new technique. It is the secondary object of the present invention to develop a technique for adjusting anchoring properly to thereby provide a polymer dispersion type liquid crystal display element with improved response to electric field and improved optical hysteresis. Further, it is the tertiary object of the present invention to provide an improved polymer dispersion type liquid crystal display element based on the above-mentioned two objects.
It is noted that although the present invention as a group is on the basis of the same or similar conception, since the each individual invention is embodied into different examples, the present invention as a group is divided into the first inventive group; the second inventive group; the third inventive group; and the fourth inventive group for every closely related invention in the specification. Hereinafter, the description on the content of the invention is given in sequence for every group (inventive group).
According to the first inventive group, the relationship between manufacturing conditions (polymerization temperature, intensity of ultraviolet and ultraviolet irradiation time) and optical hysteresis is determined, on the basis of which the optical hysteresis of the liquid crystal display element is reduced.
The first inventive group comprises following aspects:
(1) A polymer dispersion type liquid crystal display element, in which a polymer dispersion type liquid crystal is sandwiched between a pair of substrates each having an electrode at the inside thereof,
wherein said polymer dispersion type liquid crystal is such that liquid crystal droplets are dispersed and held in a continuous phase of matrix comprising polymer compound or are dispersed and held in networks of a three dimensional network form of matrix comprising polymer compound, and
wherein said liquid crystal droplets located in all areas except an area in the vicinity of interfaces between said substrates and said polymer dispersion type liquid crystal are substantially identical to each other in shape and size.
(2) In the above described aspect (1), standard deviation in average particle size of said liquid crystal droplets is within the range of xc2x15% of a mean value.
(3) In the above described aspect (1) or (2), wherein said polymer compound comprises polymers including monofunctional acrylate and/or multifunctional acrylate.
(4) In the above described aspect (3), said monofunctional acrylate is isostearyl acrylate; and said multifunctional acrylate is at least one material selected from the group consisting of triethylene glycol diacrylate, PEG#200 diacrylate, PEG#400 diacrylate, neopentyl glycol diacrylate, 1,6-hexandiol diacrylate, trimethylolpropane triacrylate, pentaerythlytoltriacrylate, and bifunctional urethane acrylate expressed by the chemical formula 1 given below:
CH2xe2x95x90CHCOOxe2x80x94Rxe2x80x2xe2x80x94OOCNHxe2x80x94(Rxe2x80x94NHCOO-(polyol)-OOCNH)nxe2x80x94Rxe2x80x94NHCOOxe2x80x94Rxe2x80x2xe2x80x94OCOCHxe2x95x90CH2xe2x80x83xe2x80x83Chemical formula 1
where n=an integer.
(5) A method for producing a polymer dispersion type liquid crystal display element, said method comprising a phase separation step in which after a liquid crystal polymer precursor compatible solution including liquid crystal and polymer precursor is placed between a pair of substrates each having an electrode at the inside thereof, a surface of said substrates is irradiated with ultraviolet so that said liquid crystal and said polymer precursor in said liquid crystal polymer precursor compatible solution can be phase-separated from each other and also said polymer precursor can be polymerized and cured, to thereby produce a polymer dispersion type liquid crystal in which liquid crystal droplets are dispersed and held in a continuous phase of matrix comprising polymer compound or are dispersed and held in networks of a three dimensional network form of matrix comprising polymer compound, said phase separation step comprising the step of controlling the time T from initiation of the irradiation of ultraviolet until completion of the phase separation so that at least any one of a degree of polymerization of said polymer precursor of said liquid crystal polymer precursor compatible solution, a rate of phase separation and a generating density of liquid crystal nuclei separated can be controlled to even particle sizes of the liquid crystal droplets dispersed and held in said matrix.
(6) In the above described aspect (5), where T1 is the time from said irradiation of ultraviolet until the initiation of phase separation of said liquid crystal polymer precursor compatible solution and T10-90 is the time required for a rate of progress of phase separation to change from 10% to 90% when the rate of progress of the phase separation for all liquid crystals to be separated from said liquid crystal polymer precursor compatible solution is defined as 100%, the control of said time T is performed by controlling said time T1, said time T10-90, or both of said time T1 and T10-90.
(7) In the above described aspect (5), temperature of said liquid crystal polymer precursor compatible solution and intensity of ultraviolet with which said liquid crystal polymer precursor compatible solution is irradiated are controlled so that the relation of T10-90=axc3x97T1+b (a, b are constants of a linear function) can hold between said time T1 and said time T10-90 and said a can be within the range of from 0.4 or more to 0.7 or less.
(8) In the above described aspect (6), said time T1 is controlled to be 5 seconds or less by controlling temperature of said liquid crystal polymer precursor compatible solution and intensity of ultraviolet with which said liquid crystal polymer precursor compatible solution is irradiated.
(9) In the above described aspect (8), said intensity of ultraviolet is not less than 100 mW/cm2.
(10) In the above described aspect (8), said intensity of ultraviolet is not less than 100 mW/cm2 and also said temperature of said liquid crystal polymer precursor compatible solution is higher than thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 2 to 15xc2x0 C.
(11) In the above described aspect (8), said intensity of ultraviolet is in the range of 160 mW/cm2 to 400 mW/cm2 and also said temperature of said liquid crystal polymer precursor compatible solution is higher than thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 6 to 13xc2x0 C.
(12) In the above described aspect (6), temperature of said liquid crystal polymer precursor compatible solution and intensity of ultraviolet with which liquid crystal polymer precursor compatible solution is irradiated are controlled so that said time T10-90 can be 6 seconds or less.
(13) In the above described aspect (12), said intensity of ultraviolet is not less than 100 mW/cm2.
(14) In the above described aspect (12), said intensity of ultraviolet is not less than 100 mW/cm2 and also said temperature of said liquid crystal polymer precursor compatible solution is higher than thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 2 to 15xc2x0 C.
(15) In the above described aspect (12), said intensity of ultraviolet is in the range of 160 mW/cm2 to 400 mW/cm2 and also said temperature of said liquid crystal polymer precursor compatible solution is higher than thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 6 to 13xc2x0 C.
(16) In the above described aspect (6), said time T1 and said time T10-90 are controlled to be 5 seconds or less and 6 seconds or less, respectively, by controlling temperature of said liquid crystal polymer precursor compatible solution and intensity of ultraviolet with which said liquid crystal polymer precursor compatible solution is irradiated.
(17) In the above described aspect (5) or (16), said liquid crystal polymer precursor compatible solution includes monofunctional acrylate and/or multifunctional acrylate.
(18) In the above described aspect (17), said monofunctional acrylate is isostearyl acrylate; and said multifunctional acrylate is at least one material selected from the group consisting of triethylene glycol diacrylate, PEG#200 diacrylate, PEG#400 diacrylate, neopentyl glycol diacrylate, 1,6-hexandiol diacrylate, trimethylolpropane triacrylate, pentaerythlytoltriacrylate, and bifunctional urethane acrylate expressed by the chemical formula 1 given below:
CH2xe2x95x90CHCOOxe2x80x94Rxe2x80x2xe2x80x94OOCNHxe2x80x94(Rxe2x80x94NHCOO-(polyol)-OOCNH)nxe2x80x94Rxe2x80x94NHCOOxe2x80x94Rxe2x80x2xe2x80x94OCOCHxe2x95x90CH2xe2x80x83xe2x80x83Chemical formula 1
where n=an integer.
Next, the significance of the constructions described above will be described below.
Shown in FIG. 1 is an example of measured optical hysteresis (ambient temperature of 30xc2x0 C. when measured) of a conventional polymer dispersion type liquid crystal display element. In FIG. 1, a solid line represents a transmittance curve at a raised voltage (an applied voltage is gradually raised) and a broken line represents a transmittance curve at a dropped voltage (an applied voltage is gradually dropped). As shown in FIG. 1, a general type of polymer dispersion type liquid crystal display element has a strong optical hysteresis. The term of xe2x80x9coptical hysteresisxe2x80x9d used herein is intended to mean the property of causing a difference in transmittance between in the process of the voltage being raised and in the process of the voltage being dropped when the same voltage is applied, and the magnitude is represented by percentage for luminance of white level.
Incidentally, in the polymer dispersion type liquid crystal, there are two orientation patterns of liquid crystal molecules in the liquid crystal droplets to be oriented in the direction of the major axes (Sov. Phys. JETP58(6), December 1983). One of the orientation patters is the bipolar form having two poles. In the bipolar-form orientation pattern, each liquid crystal molecule in the droplet is oriented toward two poles, with the major axis being in parallel to the interfacial boundary (spherical surface). Another one is the radial form. In the radial-form orientation pattern, a single pole (point defect) exists in the vicinity of the center of liquid crystal droplet and each liquid crystal molecule in the droplet is oriented in the radial direction, with one end of the major axis oriented toward said single pole and the other oriented to the spherical surface.
In general, the polymer dispersion type liquid crystal display element exhibits strong optical hysteresis in the relation to operating temperature. The origin of this hysteresis effect will be examined in association with the orientation patterns mentioned above. It is known that the polymer dispersion type liquid crystal display element has a tendency of the optical hysteresis increasing excessively at temperatures lower than a certain temperature. The origin is thought to be due to the transition of the orientation pattern of the liquid crystal molecules from the bipolar form to the radial form when temperature of the element becomes lower than a certain temperature. Also, it is thought that the primary origin of the optical hysteresis caused when the temperature for the bipolar-form orientation pattern to be produced exceeds the certain temperature is due to displacement of the poles in the bipolar-form orientation pattern or disappearance of the poles being caused by variations in applied voltage (e.g. Liquid Crystal Dispersions written by P. S. Drzaic at page 269, World Scientific 1996). On the other hand, in the radial-form orientation pattern of liquid crystals, there inevitably exist some liquid crystal molecules orienting in a direction perpendicular to the substrates while no voltage is applied as well. Due to this, in the polymer dispersion type liquid crystal element in which liquid crystal molecular orientation is controlled to make the switching between scattering condition and permeation, the liquid crystal droplets are required to take the bipolar-form orientation pattern, from the view point of contrast.
In short, reduction of the optical hysteresis requires that the liquid crystal molecules take the bipolar-form orientation pattern and also the bipolar-form orientation pattern is stably maintained in an operating temperature range of the liquid crystal display element (a driving temperature range of the element). As illustrated in the experiments discussed later, the inventors found out the fact that generation of oriented poles and transition of the poles caused by the application of voltage are highly dependent on the shape of liquid crystal droplet and a magnitude of interfacial restrictive force (anchoring) of the polymer surrounding the liquid crystal droplets.
Specifically, the more the form of the liquid crystal droplets nears to a low-distortion ellipsoid of revolution, the more the movement of the poles decreases, whereby the orientation pattern is stabilized and the optical hysteresis is reduced. In particular, when variability of the particle size of the liquid crystal droplets is within the range of 10%, the optical hysteresis is reduced drastically. However, because of wettability between the substrates and the liquid crystals, the liquid crystal droplets in the vicinity of the substrates come into semi-spherical in shape, with their great circle contacting with the substrates. It is thus difficult to control the shape of the liquid crystal droplets contacting with the substrates, so the remaining liquid crystal droplets except those contacting with the substrates should be controlled to be formed into a substantial ellipsoid of revolution form. Also, as long as the liquid crystal droplets each have a substantial ellipsoid of revolution, the liquid crystal droplets may be partially linked with another liquid crystal droplets.
The higher the percentage of the liquid crystal content in liquid crystal polymer precursor compatible solution in preparation of the polymer dispersion type liquid crystal, the more outstandingly the optical hysteresis appears. This is a consequence of: with a higher percentage of the liquid crystal content, the particle size of the liquid crystal droplets increases in general to facilitate distortion of the liquid crystal droplets, which in turn allows extra poles (more than two poles) to occur.
When the interfacial anchoring strength is weak, even if there exists an extra pole, since the extra pole other than the poles in the bipolar orientation is destroyed with the application of an electric field, the optical hysteresis is weakened. However, any weak anchoring strength is not always of desirable. This is because, in the case of significantly weak anchoring strength, the liquid crystal molecules do not go back to their original state even with de-energization of the applied voltage, which in turn makes it difficult to do the light-and-dark switching by the control of the application of voltage. Therefore, the anchoring strength must be adjusted to a proper strength. In general, the optical hysteresis in the high temperature range is antinomic with that in the low temperature range. There is the tendency that when the optical hysteresis in the high temperature range is tried to be reduced, the optical hysteresis in the low temperature range increases, and vise versa. Due to this, the prior arts have not succeeded in accomplishing the polymer dispersion type liquid crystal display element having the optical hysteresis which is capable to be reduced in a wide temperature range.
Next, the ways of reducing distortion of the liquid crystal droplets to adjust the form of the liquid crystal droplets will be described below. The first way is to accelerate a rate of polymerization of the polymer when phase separation and polymerization of the polymer are made by irradiation of ultraviolet to the liquid crystal polymer precursor compatible solution.
Description on this will be given with reference to FIG. 2. FIG. 2 schematically illustrates the forming states of liquid crystal droplets: FIG. 2(a) illustrates the form of the liquid crystal droplets in the case of the rate of polymerization being fast (e.g. the rate of polymerization not more than 6 sec.) and FIG. 2(b) illustrates the form of the liquid crystal droplets in the case of the rate of polymerization being slow (e.g. the rate of polymerization not less than 10 sec.). With an accelerated rate of polymerization, the liquid crystal droplet separating nuclei (microscopic liquid crystal droplets produced immediately after the phase separation) are allowed to spread uniformly, which in turn can allow the separating nuclei to grow to liquid crystal droplets within a short time. Consequently, neighboring liquid crystal droplets are kept with properly spaced intervals from each other, and liquid crystal droplets with relatively uniform form are formed. On the other hand, with a decelerated rate of polymerization, it take lots of time until completion of polymerization, which can allow other liquid crystal droplets to squeeze in spaces between the neighboring liquid crystals in stages of growth of the liquid crystal droplets, to cause mutual forms of the droplets to be distorted and thereby form liquid crystal droplets of uneven in shape. In addition, with the decelerated rate of polymerization, excessively large liquid crystal droplets can be produced. As discussed later in detail, the rate of polymerization of the polymer precursor can be controlled by adjusting temperature of the liquid crystal polymer precursor compatible solution (hereinafter it is referred to as xe2x80x9cpolymerization temperaturexe2x80x9d) and intensity of irradiation of ultraviolet (hereinafter it is simply referred to as xe2x80x9cintensity of ultravioletxe2x80x9d) during the phase separation.
The second way is to adjust viscosity and hardness of the polymer precursors surrounding the separating nuclei during the separation. When the polymer precursors are low in degree of polymerization, their viscosity is low and their hardness remains soft, so that the separating nuclei are affected, during growth, by fluctuation of the liquid crystal polymer precursor compatible solution and thus are liable to develop into a distorted form. On the other hand, when the separating nuclei separate in a stage in the full development of polymerization of the polymer precursors, the growth of the separating nuclei is restricted by the polymer precursors high in viscosity and high in hardness (e.g. dimer or trimer), and the separating nuclei avoid high-hardness portions of the polymer precursor, during growth. As a result of this, distorted liquid crystal droplets are formed.
It will be appreciated from the above that there exists an optimum range in the degree of polymerization of the polymer precursors (which corresponds to the hardness of the polymer precursors) during a stage of separation of liquid crystals. It should be noted that the hardness of the polymer precursors can be adjusted, at the time of separation of the separating nuclei, by either controlling the degree of the polymerization of the polymer precursors properly or selecting type and composition of the polymer precursor properly.
The third way is to control a generating density of separating liquid crystal nuclei in a proper manner. FIG. 3(a) is a schematic showing of separating liquid crystal nuclei of a high generating density and FIG. 3(b) is a schematic showing of separating liquid crystal nuclei of a low generating density. When the generating density of the nuclei is excessively high, the separating nuclei contact or connect to each other in the process of growth to cause distortion of the nuclei. On the other hand, when the generating density of the nuclei is excessively low, the separating nuclei matures into excessively large liquid crystal droplets, while the number of liquid crystal droplets decrease, leading to deterioration of scattering characteristics. For this reason, it is necessary to limit the generating density of the nuclei of the liquid crystal droplets to an optimum range. The generating density of the nuclei is dependent on factors such as polymerization temperature, a rate of polymerization, degree of polymerization of polymer precursors, and intensity of ultraviolet. Hence, the generating density of the separating nuclei can be controlled by adjusting the polymerization temperature and the intensity of irradiation of ultraviolet. In addition, the generating density can be also controlled by setting the compositions of the liquid crystal polymer precursor compatible solution properly.
As described above, the adjustment of rate of polymerization, degree of polymerization and generating density of the separating nuclei can allow the liquid crystal droplets to have uniform form and also allow the optical hysteresis to be reduced. The factors above can be controlled by varying the polymerization temperature and the intensity of ultraviolet. However, there is a few precedents for the study of the degree of polymerization and the rate of polymerization in the phase separation having been made. Among others, there is no precedent for the study of the optical hysteresis characteristics of the liquid crystal display element having been made from the viewpoints of rate of polymerization, degree of polymerization, and generating density of the separating nuclei.
Accordingly, the inventors have introduced capacitance as a physical value which reflects the behavior of liquid crystal molecules more directly in its relation with the applied voltage, developing the technique of estimating the rate of polymerization, the degree of polymerization and generating density of the separating nuclei by use of the capacitance. And, they have succeeded in forming the polymer dispersion type liquid crystal display element which displays low optical hysteresis in a wide temperature range, based on the results obtained by the developed technique. The liquid crystal display element thus formed have performance which has never been accomplished by conventional techniques, specifically, stable display performance in a wide temperature range.
The significance of capacitance as the physical value is described below. When the liquid crystal polymer precursor compatible solution injected in between a pair of substrates (hereinafter they are referred to as xe2x80x9cliquid crystal panelxe2x80x9d) is irradiated with ultraviolet, with a bias voltage being applied thereto, the separating liquid crystal nuclei are formed by phase separation, and at the same time as the form of the separating nuclei, the liquid crystal molecules in the separating nuclei rise in response to the bias voltage. Thus, the capacitance of the liquid crystal panel varies in response to the form of the separating nuclei and the growth (which means increase in the amount of separating liquid crystals). Hence, the proportion of separating liquid crystals (to the total separating amounts) at some point in the progress of phase separation can be grasped by measuring the capacitance in the progress of phase separation with increased time. As long as the liquid crystal polymer precursor compatible solution is identical in composition and also the polymerization temperature (temperature of liquid crystal panel) is constant, the separation of liquid crystals in the phase separation is determined by the degree of progress of polymerization of the polymer precursors. Accordingly, the degree of progress of polymerization (the degree of polymerization) and the rate of polymerization can be found by estimating the proportion of separating liquid crystal by the capacitance. Further, this will be specifically described with reference to experiments.
(Experiment 1)
In Experiment 1, the significance of measuring the capacitance is experimentally clarified. In this experiment, the liquid crystal panel similar to that prepared in Example 1-1 as described later (in which liquid crystal polymer precursor compatible solution is filled), which was set at a temperature higher than thermal phase separation temperature of the liquid crystal polymer precursor compatible solution by 9xc2x0 C., was irradiated with ultraviolet of intensity of 200 mW/cm2, to measure the capacitance in the progress of phase separation. The methods of adjusting the liquid crystal panel temperature, of measuring the capacitance and of preparing the liquid crystal panel and other detailed conditions are described later, with reference to Example 1-1 of the first inventive group.
FIG. 4 shows the measurement results of the capacitance. As shown in FIG. 4, the capacitance did not change for a certain time (T1) from the start of ultraviolet irradiation. After the passage of the certain time, the capacitance increased sharply and thereafter was kept stable in a high level. This result shows that after the passage of the certain time (T1), the phase separation started, from the point of which the separating of the liquid crystals progressed rapidly. Additionally, the stabilized capacitance indicates the completion of the separating of liquid crystals (the completion of polymerization of polymer).
As shown in FIG. 4, where the time from the irradiation before the start of phase separation is T1, the time from the start of phase separation before the completion thereof is T2, and the time required for the capacitance to change from 10% to 90% is T10-90, when the T2 is estimated by the T10-90 go, the time T1 corresponds to the time from the start of irradiation of ultraviolet before the start of phase separation and the time T10-90 corresponds to the rate of polymerization. It is to be noted that the reason for letting the time T2 be T10-90, not T0-100, is that letting T2=T0-100 leads to increase in error of measurement.
In general, the intenser the intensity of ultraviolet, the faster the rate of polymerization of a polymer precursor becomes. Accordingly, with increasing intensity of ultraviolet, the time T1, T2, particularly the time T2, shortens. In addition, the higher the compatible solution temperature (which is the panel temperature and the polymerization temperature) as compared with thermal phase separation temperature of the liquid crystal polymer precursor compatible solution, the more frequently the phase separation does not occur until the polymerization of the polymer precursor progresses to some extent. As a result, the time T1 from after the start of irradiation of ultraviolet before the start of phase separation lengthens. In other words, the lengthening time T1 means that polymerization of the polymer precursor is being in progress before the phase separation starts, which in turn means that at the time of separating of liquid crystals, the polymer precursor grows into polymer such as dimer or trimer. Polymer is high in viscosity and in hardness, as compared with monomer, so that as the time T1 increases, the viscosity and hardness of the polymer precursor increase and the generating density of the separating liquid crystal nuclei decreases. This means that as the time T1 increases, deformed liquid crystal droplets are easily produced increasingly, as mentioned above.
Further, there is a close relation between the polymerization temperature of liquid crystal polymer precursor compatible solution and the rate of polymerization, also. The higher the polymerization temperature, the longer the time T2 becomes.
It will be understood from the above that the measurements of the time T1 and T2 enable a desirable phase separation condition for forming the well-formed liquid crystal droplets to be determined, in association with the intensity of ultraviolet and the liquid crystal panel temperature (polymerization temperature).
(Experiment 2)
In Experiment 2, the relationship between the degree of polymerization (viscosity.hardness) of the polymer precursor and the degree of deformation of the liquid crystal droplets (the factor on which the optical hysteresis is dependent), the relationship between the intensity of ultraviolet and the optical hysteresis, and the relationship between the intensity of ultraviolet and the optical hysteresis under the condition of constant polymerization temperature are clarified. Other conditions than the intensity of ultraviolet and the polymerization temperature are the same as those in the above Example 1, unless otherwise specified.
First of all, the relationship between the polymerization temperature and the optical hysteresis is described with reference to FIG. 5 (as conceptually illustrated). As already discussed, the polymerization temperature, close to the thermal phase separation temperature of liquid crystal polymer precursor compatible solution, causes the phase separation easily, and as such can allow liquid crystal separating nuclei to separate out in the polymer precursor low in the degree of polymerization. As a result of this, although the generating density of the separating nuclei increases, since the viscosity.hardness of the polymer precursor is low, the separating droplets are linked to each other in the stage of growth to thereby produce the distorted liquid crystal droplets. On the other hand, when the polymerization temperature is high, the phase separation does not occur until he polymerization of the polymer precursor progresses to some extent. Thus, as a consequence of the phase separation occurring at a stage of the polymerization progressing to some extent, the generating density of the separating nuclei is reduced, and since the viscosity.hardness of the polymer precursor around the separating nuclei is high, the liquid crystal droplets are rendered prone to deformation. Thus, in either case, the polymer dispersion type liquid crystal in which deformed liquid crystal droplets are dispersed is formed, resulting in increase in optical hysteresis. In view of the above, it is necessary to find out a proper polymerization temperature that enables the optical hysteresis to decrease.
FIG. 6 shows the measurement results of the optical hysteresis resulting from changes in the intensity of ultraviolet and the polymerization temperature for the liquid crystal panel (details are described later in Example 1-5). In this measuring experiment, the liquid crystal polymer precursor compatible solution whose thermal phase separation temperature is about 10xc2x0 C. is used.
It will be understood from the result shown in FIG. 6 that at the intensity of ultraviolet of 110 mW/cm2 (♦xe2x80x94♦), the lower the polymerization temperature, the smaller the hysteresis becomes. It will be also appreciated that at the intensity of ultraviolet of 200 mW/cm2, 300 mW/cm2, 400 mW/cm2, and 550 mW/cm2, there exist polymerization temperatures in which the optical hysteresis takes minimum values, and there is a tendency that with increasing intensity of ultraviolet, the polymerization temperatures coming to the minimum shift to higher temperatures. Further, the weaker the intensity of ultraviolet, the larger the optical hysteresis. This means that the weak intensity of ultraviolet leads to decrease in the rate (progress speed) of phase separation after the start of phase separation, and the high polymerization temperature leads to increase in the viscosity.hardness and in turn leads to deceleration in the progress speed of the phase separation, so that, in any case, the distortion of the liquid crystal droplets increases and resultantly the optical hysteresis increases.
In FIG. 6, the temperatures in which the optical hysteresis takes the minimum values range from 12xc2x0 C. (at 110 mW/cm2) to 23xc2x0 C. (at 400 mW/cm2 and 550 mW/cm2) in the polymerization temperature, with the minimum values ranging from 16xc2x0 C. to 23xc2x0 C. Also, it will be understood that the intensity of ultraviolet of not less than 200 mW/cm2 is required for achieving the optical hysteresis of not more than 1%. It is noted that the 23xc2x0 C. is the temperature higher than the thermal phase separation temperature (10xc2x0 C.) by 13xc2x0 C.
Shown in FIG. 12 is the result of FIG. 6 presented by the connection between the intensity of ultraviolet and the polymerization temperature at which the optical hysteresis is reduced to the minimum. The liquid crystal display element having small hysteresis can be obtained by setting the polymerization temperature and the intensity of ultraviolet properly based on FIG. 12. However, it is preferable to set the intensity of ultraviolet to be in the range of 110 mW/cm2 or more to 400 mW/cm2 or less, to allow for photodecomposition of the liquid crystals as well as the optical hysteresis reduction effect in the low temperature range.
From the above result it is understood that the optical hysteresis can be reduced significantly by selecting a temperature in the vicinity of the polymerization temperature in which the optical hysteresis is reduced to the minimum, according to the intensity of ultraviolet, to perform the phase separation.polymerization.
(Experiment 3)
In Experiment 3, the liquid crystal panels were prepared, with intensity of ultraviolet of 200 mW/cm2 irradiated to the liquid crystal panels in common but changes in the polymerization temperature only. The liquid crystal panel thus prepared was measured in respect of the time T1 and the time T2 in association with the polymerization temperature. The remaining manufacturing conditions are the same as those of Example 1-1 described later.
FIG. 7 shows the measurement results. As apparent from FIG. 7, with rising polymerization temperature, the time T1 and the time T2 both generally lengthened. That the higher the polymerization temperature, the longer the time T1 means that the phase separation is started in a stage in which the polymerization of polymer precursor is in progress. Also, the time T2 from the start of the phase separation to the completion of the phase separation lengthened more at higher polymerization temperatures than at lower polymerization temperatures. That is presumed to be because the polymerization progresses considerably before the start of phase separation, to increase the viscosity, which resultantly retards the polymerization reaction after the start of the phase separation.
(Experiment 4)
Further, various kinds of elements were prepared, with the polymerization temperature kept constant at 13xc2x0 C. and with the remaining conditions being in common with those in the case of FIG. 6 but changes in the intensity of ultraviolet only. The elements thus prepared were measured in respect of the connection between the intensity of ultraviolet and the optical hysteresis. The results are shown in FIG. 8. It is understood from FIG. 8 that at an increasing intensity of ultraviolet, the element can have reduced optical hysteresis, and at the intensity of ultraviolet of 100 mW/cm2 or more, the optical hysteresis is brought to 1.5% or less. In detail, at the intensity of ultraviolet of 200 mW/cm2, for example, the optical hysteresis is brought to about 1%, and at intensity of ultraviolet of 300 mW/cm2 and of 500 mW/cm2, the optical hysteresis characteristics of the liquid crystal display elements are brought to about 0.8% and to about 0.3%, respectively.
It is thought that the reason why at increasing intensity of ultraviolet, the optical hysteresis has a tendency to be reduced is that with increasing intensity of ultraviolet, the rate of polymerization increases to contribute rapid growth of the liquid crystal droplets, and as such can allow the liquid crystal droplets to be small in distortion and equal in size. To ensure this effect of the intensity of ultraviolet, T2 (equivalent to the rate of polymerization) which is determined by combination of the intensity of ultraviolet with the polymerization temperature can be measured.
(Experiment 5)
In Experiment 5, the liquid crystal panels, prepared with various changes in the intensity of ultraviolet as well as in the polymerization temperature, were measured in respect of capacitance in the same manner as in Experiment 1, to determine the time T1 and the time T2 (the time T2 defined by 10%-90%), on the basis of which the relationship between the time T1 and the time T2 was clarified.
Shown in FIG. 9 are the measurement results in the relationship between the time T2 and the optical hysteresis. From FIG. 9 it can be seen that when the time T2 is 6 seconds or less, the optical hysteresis is brought to about 2% or less; that when the time T2 is 4.5 seconds or less, the optical hysteresis is brought to about 1.4% or less; and that the time T2 is required to be set 2.5 seconds or less, in order to bring the optical hysteresis to about 0.8% or less. It is also seen that the time T2 is required to be set 1.5 seconds or less, in order to bring the optical hysteresis to 0.5% or less.
Shown in FIG. 14 are the measurement results of Experiment 5 in the relationship between the time T1 and the time T2. It can be seen from FIG. 14 that there is a generally regular correlation between the time T1 and the time T2, so that they can make approximations with T2=axc2x7T1+b (a, b are constants of a linear function). And, it became clear that the number a in the linear function has not so much dependence on the intensity of ultraviolet and is estimated to be 0.4 or more to 0.7 or less.
The results of FIG. 14 are thought to indicate that when the time T1 from the start of irradiation of ultraviolet before the start of phase separation is long, the polymerization of polymer precursor progresses before the start of phase separation, which allows the solution to increase in viscosity and in hardness, with the result that the time T2 (the rate of polymerization) is rendered slow.
The rate of polymerization can be regulated also by changing the composition of the polymer precursor compositions including an additive by addition of a polymerization promoter, for example.
The inventors confirmed that the rate of phase separation was changed by forming an insulation layer at the interface of the substrate. That is thought to be because the liquid crystals separated out in the vicinity of the interface of the substrate are affected by surface tension of the substrate. The larger the surface tension to the liquid crystals, the sooner the liquid crystals are separated out from the compatible solution.
(2) 2nd inventive group
It is the primary object of the invention of the second inventive group that the relation of manufacturing conditions for the phase separation of polymer dispersion type liquid crystals (polymerization temperature, intensity of ultraviolet and ultraviolet irradiation time) with particle size of the liquid crystal droplets, orientation transition temperature of the liquid crystal molecules, tilt angles of the liquid crystal molecules and anchoring strength and the relationship between these and the optical hysteresis are determined, on the basis of which the optical hysteresis of the liquid crystal display element is reduced drastically in the operating temperature range of the liquid crystal display element (temperature of the element during driving), in a low temperature in particular, to thereby produce a satisfactory display performance of the liquid crystal display element.
In the second inventive group, the anchoring index defined by the following expression 2-1 is introduced as an index to estimate a magnitude of the anchoring strength, for making use of the anchoring index.
Anchoring index=(V90xc3x97R)/dxe2x80x83xe2x80x83Expression 2-1,
where
V90: an applied voltage required for the transmittance to become 90% in the temperature of element of 30xc2x0 C.;
d (xcexcm): an interval between the substrates; and
R (xcexcm): an average particle size of a liquid crystal droplet or an average interval of a three dimensional network form of matrix comprising polymer compound.
The second inventive group comprises the following aspects.
(19) A polymer dispersion type liquid crystal display element in which a polymer dispersion type liquid crystal is sandwiched between a pair of substrates each having an electrode at the inside thereof,
wherein said polymer dispersion type liquid crystal is such that liquid crystal droplets are dispersed and held in a continuous phase of matrix comprising polymer compound or are dispersed and held in networks of a three dimensional network form of matrix comprising polymer compound,
wherein liquid crystal molecules in said liquid crystal droplets present a bipolar-form orientation pattern having at least two poles in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, while no voltage is applied to said electrodes, and
wherein, where a clear point transition temperature of said liquid crystal is let be Tni, said bipolar-form orientation pattern is maintained at least when an operating temperature of the element falls in the range of from 5xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(20) In the above described aspect (19), tilt angles of said liquid crystal molecules, in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, to said interfaces are not more than 10 degrees, while no voltage is applied to said electrodes.
(21) In the above described aspect (19), said bipolar-form orientation pattern is maintained under an operating temperature of said element falling in the range of from 0xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(22) In the above described aspect (21), tilt angles of said liquid crystal molecules, in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, to said interfaces are not more than 10 degrees, while no voltage is applied to said electrodes.
(23) In the above described aspect (19), said bipolar-form orientation pattern is maintained under an operating temperature of said element falling in the range of from xe2x88x925xc2x0C. to (Tnixe2x88x925)xc2x0C.
(24) In the above aspect (23), tilt angles of said liquid crystal molecules, in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, to said interfaces are not more than 10 degrees, while no voltage is applied to said electrodes.
(25) In the above aspect (19), said liquid crystal droplets located in all areas except an area in the vicinity of interfaces between said substrates and said polymer dispersion type liquid crystal are substantially identical to each other in shape and size.
(26) In the above aspect (25), variations in size of said liquid crystal droplets are within 10%.
(27) In the above aspect (19), said liquid crystal droplets located in all areas except an area in the vicinity of interfaces between said substrates and said polymer dispersion type liquid crystal are substantially identical to each other in shape and size, and wherein tilt angles of said liquid crystal molecules, in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, to said interfaces are not more than 10 degrees, while no voltage is applied to said electrodes.
(28) In the above aspects (19) to (27), said polymer compound comprises polymers including monofunctional acrylate and/or multifunctional acrylate.
(29) In the above described aspect (28), said monofunctional acrylate is isostearyl acrylate; and said multifunctional acrylate is at least one material selected from the group consisting of triethylene glycol diacrylate, PEG #200 diacrylate, PEG#400 diacrylate, neopentyl glycol diacrylate, 1,6-hexandiol diacrylate, trimethylolpropane triacrylate, pentaerythlytoltriacrylate, and bifunctional urethane acrylate expressed by the chemical formula 1 given below:
CH2xe2x95x90CHCOOxe2x80x94Rxe2x80x2xe2x80x94OOCNHxe2x80x94(Rxe2x80x94NHCOO-(polyol)-OOCNH)nxe2x80x94Rxe2x80x94NHCOOxe2x80x94Rxe2x80x2xe2x80x94OCOCHxe2x95x90CH2xe2x80x83xe2x80x83Chemical formula 1
where n=an integer.
(30) A polymer dispersion type liquid crystal display element in which a polymer dispersion type liquid crystal is sandwiched between a pair of substrates each having an electrode at the inside thereof,
wherein said polymer dispersion type liquid crystal is such that liquid crystal droplets are dispersed and held in a continuous phase of matrix comprising polymer compound or are dispersed and held in networks of a three dimensional network form of matrix comprising polymer compound, and
wherein a value of (V90 (volt)XR)/d is 0.7 or more, where V90 is an applied voltage required for transmittance of a voltage transmittance characteristic of said polymer dispersion type liquid crystal display element to become 90% under 30xc2x0 C. of the temperature of element; d (xcexcm) is an interval between said pair of substrates; and R (xcexcm) is an average particle size of said liquid crystal droplets.
(31) In the above described aspect (28), liquid crystal molecules in said liquid crystal droplets present a bipolar-form orientation pattern having at least two poles in the vicinity of interfaces between said liquid crystal droplets and said polymer compound.
(32) In the above described aspect (31), tilt angles of said liquid crystal molecules, in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, to said interfaces are not more than 10 degrees, while no voltage is applied to said electrodes.
(33) In the above described aspect (31), a clear point transition temperature of said liquid crystal is let be Tni, said bipolar-form orientation pattern is maintained at least when operating temperature of said element falls in the range of from 5xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(34) In the above described aspect (33), tilt angles of said liquid crystal molecules, in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, to said interfaces are not more than 10 degrees, while no voltage is applied to said electrodes.
(35) In the above described aspect (30), where a clear point transition temperature of said liquid crystal is let be Tni, said bipolar-form orientation pattern is maintained at least when an operating temperature of said element falls in the range of from 0xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(36) In the above described aspect (35), tilt angles of said liquid crystal molecules, in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, to said interfaces are not more than 10 degrees, while no voltage is applied to said electrodes.
(37) In the above described aspect (31), where a clear point transition temperature of said liquid crystal is let be Tni, said bipolar-form orientation pattern is maintained when an operating temperature of said element falls in the range of from xe2x88x925xc2x0 C. to (Tni-5)xc2x0C.
(38) In the above described aspect (37), tilt angles of said liquid crystal molecules, in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, to said interfaces are not more than 10 degrees, while no voltage is applied to said electrodes.
(39) A method for producing a polymer dispersion type liquid crystal display element, said method comprising the phase separation step in which after a liquid crystal polymer precursor compatible solution including liquid crystal and polymer precursor is placed between a pair of substrates each having an electrode at the inside thereof, said substrates is irradiated on their surface with ultraviolet so that said liquid crystal and said polymer precursor in said liquid crystal polymer precursor compatible solution can be phase-separated from each other to thereby produce a polymer dispersion type liquid crystal in which liquid crystal droplets are dispersed and held in a polymer matrix, wherein temperature of said liquid crystal polymer precursor compatible solution at the time of said irradiation of ultraviolet is rendered higher than a thermal phase separation temperature of said liquid crystal polymer precursor compatible solution.
(40) In the above described aspect (39), said temperature of said liquid crystal polymer precursor compatible solution is rendered higher than said thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 2 to 15xc2x0 C.
(41) In the above described aspect (39), intensity of ultraviolet irradiation is set to be not less than 160 mW/cm2.
(42) In the above described aspect (39), said temperature of said liquid crystal polymer precursor compatible solution is rendered higher than said thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 6 to 13xc2x0 C. and also said intensity of ultraviolet irradiation is set at 160 mW/cm2 to 400 mW/cm2.
(43) A method for producing a polymer dispersion type liquid crystal display element in which a polymer dispersion type liquid crystal is sandwiched between a pair of substrates each having an electrode at the inside thereof, said polymer dispersion type liquid crystal being such that liquid crystal droplets are dispersed and held in a continuous phase of matrix comprising polymer compound or are dispersed and held in networks of a three dimensional network form of matrix comprising polymer compound, wherein liquid crystal molecules in said liquid crystal droplets present a bipolar-form orientation pattern having at least two poles in the vicinity of interfaces between said liquid crystal droplets and said polymer compound, while no voltage is applied to said electrodes, and wherein, where a clear point transition temperature of said liquid crystal used in said polymer dispersion type liquid crystal display element is let be Tni, said bipolar-form orientation pattern is maintained under an operating temperature of said element falling in the range of from 5xc2x0 C. to (Tnixe2x88x925)xc2x0C., said method comprising the step that under the condition that said liquid crystal polymer precursor compatible solution including liquid crystal and polymer precursor placed between said pair of substrates is kept at a higher temperature than a thermal phase separation temperature of said liquid crystal polymer precursor compatible solution, said liquid crystal polymer precursor compatible solution is irradiate with ultraviolet to allow said liquid crystal and said polymer compound to be phase-separated from each other.
(44) In the above described aspect (39), said temperature of said liquid crystal polymer precursor compatible solution is rendered higher than said thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 2 to 15xc2x0 C. and also said intensity of ultraviolet irradiation is set at not less than 100 mW/cm2.
(45) In the above described aspect (43), said temperature of said liquid crystal polymer precursor compatible solution is rendered higher than said thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 6 to 13xc2x0 C. and also said intensity of ultraviolet irradiation is set at 160 mW/cm2 to 400 mW/cm2.
(46) In the above described aspects (39) to (45), said liquid crystal polymer precursor compatible solution includes monofunctional acrylate and/or multifunctional acrylate.
(47) In the above described aspect (46), said monofunctional acrylate is isostearyl acrylate; and said multifunctional acrylate is at least one material selected from the group consisting of triethylene glycol diacrylate, PEG#200 diacrylate, PEG#400 diacrylate, neopentyl glycol diacrylate, 1,6-hexandiol diacrylate, trimethylolpropane triacrylate, pentaerythlytoltriacrylate, and bifunctional urethane acrylate expressed by the chemical formula 1 given below:
CH2xe2x95x90CHCOOxe2x80x94Rxe2x80x2xe2x80x94OOCNHxe2x80x94(Rxe2x80x94NHCOO-(polyol)-OOCNH)nxe2x80x94Rxe2x80x94NHCOOxe2x80x94Rxe2x80x2xe2x80x94OCOCHxe2x95x90CH2xe2x80x83xe2x80x83Chemical formula 1
where n=an integer.
(48) A method for producing a polymer dispersion type liquid crystal display element in which a polymer dispersion type liquid crystal is sandwiched between a pair of substrates each having an electrode at the inside thereof, said polymer dispersion type liquid crystal being such that liquid crystal droplets are dispersed and held in a continuous phase of matrix comprising polymer compound or are dispersed and held in networks of a three dimensional network form of matrix comprising polymer compound, wherein a value of (V90xc3x97R)/d is 0.7 or more, where V90 is an applied voltage required for transmittance of a voltage.transmittance characteristic of said polymer dispersion type liquid crystal display element to become 90% under 30xc2x0 C. of the temperature of element; d is an interval between said pair of substrates; and R is an average particle size of said liquid crystal droplets, said method comprising the step that under the condition that said liquid crystal polymer precursor compatible solution including liquid crystal and polymer precursor placed between said pair of substrates is maintained at a higher temperature than a thermal phase separation temperature of said liquid crystal polymer precursor compatible solution, said liquid crystal polymer precursor compatible solution is irradiate with ultraviolet to allow said liquid crystal and said polymer precursor to be phase-separated from each other.
(49) In the above described aspect (48), said temperature of said liquid crystal polymer precursor compatible solution is rendered higher than said thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 2 to 15xc2x0 C. and also said intensity of ultraviolet irradiation is set at not less than 100 mW/cm2.
(50) In the above described aspect (48), said temperature of said liquid crystal polymer precursor compatible solution is rendered higher than said thermal phase separation temperature of said liquid crystal polymer precursor compatible solution by 6 to 13xc2x0 C. and also said intensity of ultraviolet irradiation is set at 160 mW/cm2 to 400 mW/cm2.
(51) In the above described aspects (48) to (50), said liquid crystal polymer precursor compatible solution includes monofunctional acrylate and/or multifunctional acrylate.
(52) In the above described aspect (51), said monofunctional acrylate is isostearyl acrylate; and said multifunctional acrylate is at least one material selected from the group consisting of triethylene glycol diacrylate, PEG#200 diacrylate, PEG#400 diacrylate, neopentyl glycol diacrylate, 1,6-hexandiol diacrylate, trimethylolpropane triacrylate, pentaerythlytoltriacrylate, and bifunctional urethane acrylate expressed by the chemical formula 1 given below:
xe2x80x83CH2xe2x95x90CHCOOxe2x80x94Rxe2x80x2xe2x80x94OOCNHxe2x80x94(Rxe2x80x94NHCOO-(polyol)-OOCNH)nxe2x80x94Rxe2x80x94NHCOOxe2x80x94Rxe2x80x2xe2x80x94OCOCHxe2x95x90CH2xe2x80x83xe2x80x83Chemical formula 1
where n=an integer.
(53) A polymer dispersion type liquid crystal display element in which a polymer dispersion type liquid crystal is sandwiched between a pair of substrates each having an electrode at the inside thereof, said polymer dispersion type liquid crystal being such that liquid crystal droplets are dispersed and held in a continuous phase of matrix comprising polymer compound or are dispersed and held in networks of a three dimensional network form of matrix comprising polymer compound, wherein said polymer dispersion type liquid crystal display element is so constructed that when capacitance ratio of said element is defined by Expression 3-3 given below, said capacitance ratio becomes 60% or more for a voltage required for light transmittance of said element to become 10% or more:
Capacitance ratio=(capacitance for the case of any selected voltage being applied to the element/a maximum capacitance for the applied voltage)xc3x97100xe2x80x83xe2x80x83Expression 3-3
(54) In the above described aspect (53), said maximum applied voltage is 10V or more.
Next, the significance of the constructions described above will be described below.
As described above, the inventors measured the electro-optical characteristics caused by the change in the orientation pattern in the polymer matrix and discovered the phenomenon that the optical hysteresis increases when the liquid crystals are transformed from the bipolar-form orientation pattern to the radial-form orientation pattern in low temperature. Accordingly, the transition to the radial-form orientation pattern is suppressed in relatively low temperatures as well, in other words, the orientation pattern of the liquid crystals is allowed to be substantially invariable within the operating temperature range of the liquid crystal display element, whereby the optical hysteresis can be drastically reduced particularly in low temperatures to accomplish a satisfactory display performance in a wide temperature range.
A mechanism for the transition of orientation to be caused is briefly described below, before the description on the suppression of the transition of orientation pattern is given.
The liquid crystal molecules at their interfaces with the polymer are oriented with given tilt angles under relatively high temperatures, while on the other hand, they are oriented with their orienting vertical to the interfaces at low temperatures. Incidentally, the orientation pattern of the liquid crystal molecules in the polymer matrix of the polymer dispersion type liquid crystal display element, in which liquid crystals are dispersed in the polymer, is strongly dependent on the orientation of the liquid crystal molecules at their interfaces with the polymer. In other words, the liquid crystals in the polymer matrix take on an orientation pattern in which elastic energy, including potential energy, of the liquid crystal molecules at the interfaces assumes a minimum. Accordingly, under relatively high temperatures, the liquid crystal molecules inside of the liquid crystal droplets take on the orientation determined by this nature, resulting in the bipolar-form orientation pattern.
On the other hand, with decreasing temperature, there arises the transition to the radial-form orientation pattern.
The inventors found out that the above-described orientation transition temperature is dependent on anchoring strength in the interfaces between the polymer and the liquid crystals, and as such can allow the orientation transition temperature to decrease by increasing the anchoring strength. It seems that that is because orientation of the liquid crystal molecules at the interfaces of the polymer and the liquid crystal adjoining each other is determined by the anchoring strength at the interfaces, so that, when the anchoring strength is weak, the molecular major axis of each liquid crystal molecule tends to rotate easily, while on the other hand, when the anchoring strength is strong, the molecular major axis is strongly restrained by the interfaces and thus the liquid crystal molecules in the vicinity of the interfacial boundary tend to have difficulties in moving.
However, existing techniques have great difficulties in measuring the anchoring strength directly. Accordingly, the inventors introduced the above-mentioned expression of (V90 (volt) R)/d as an anchoring index to estimate a magnitude of the anchoring strength, as mentioned above, where V90 is an applied voltage required for the transmittance to become 90% at temperature of element of 30xc2x0 C.; d (xcexcm) is an interval between the substrates; and R (xcexcm) is an average particle size of a liquid crystal droplet in polymer matrix, or an average interval of a three dimensional network form of matrix comprising polymer compound.
Specifically, suppose that the liquid crystal droplets having a particle size of R are formed with linking with each other in the thickness direction of a pair of substrates spaced at an interval of d, the number of liquid crystal droplets existing between the substrates can be determined by d/R. So, when V90 is divided by the d/R, that follows (V90xc3x97R)/d, which indicates an applied voltage per matrix when the transmittance is 90%. Thus, the value of (V90xc3x97R)/d decreases when the anchoring strength of the interfaces is weak, and increases when reverse. This is because the weaker the anchoring strength, the more the liquid crystal molecules tend to be oriented in the electric field direction with a smaller field intensity.
The measurement result on the correlation between the anchoring strength and the orientation transition temperature is shown in FIG. 20. This proved that reduction of a lower limit of the operating temperature range to 10xc2x0 C. or less requires (V90xc3x97R)/dxe2x89xa70.7. Similarly, reduction of a lower limit of the operating temperature range to 5xc2x0 C. or less requires (V90xc3x97R)/dxe2x89xa70.8, and reduction of a lower limit of the operating temperature range to 0xc2x0 C. or less requires (V90xc3x97R)/dxe2x89xa70.9. Thus, the liquid crystal display element responsive to a desired operating temperature range can be produced by controlling the anchoring index (anchoring strength). In particular, the polymer dispersion type liquid crystal display element usable at lower temperatures than ever can be produced by increasing the anchoring index.
Further, the inventors discovered that the anchoring strength is dependent on polymerization temperature of polymer during irradiation of ultraviolet and established the technique of setting the anchoring strength at a desired value. In general, the liquid crystal display element is produced by the following. A liquid crystal panel, in which compatible material of polymer driver and liquid crystal is injected in between a pair of spaced apart opposing glass substrates, is kept at a given temperature and is irradiated with ultraviolet, so as to allow the liquid crystals to separate by phase separation and also allow the polymer driver to be polymerized. During the irradiation of ultraviolet, the liquid crystal panel was conventionally kept at the given temperature which was substantially the same as the temperature for the phase separation to be generated. In contrast to this, when the liquid crystal panel is irradiated with ultraviolet, with its kept at a certain temperature slightly higher than the temperature for the phase separation to be generated, a phase separation line of the spinodal decomposition which indicates a temperature condition for generation of a phase separation is shifted toward higher temperature rapidly to allow the phase separation to occur when the phase separation line reaches the temperature of the liquid crystal panel. Thus, the anchoring strength can be set at any desired magnitude by controlling the above-described certain temperature (the temperature of the mixed solution of the polymer driver and the liquid crystal) during the irradiation of ultraviolet.
The variations of the anchoring strength, depending on temperatures of the liquid crystal panel during the irradiation of ultraviolet, is thought to be due to the following. The degree of polymerization of the polymer precursor around the liquid crystals separated by the phase separation varies in response to the temperature of the then liquid crystal panel. As the configuration temperature rises, development of the polymer precursor into polymer increases and resultantly the viscosity increases. In addition, the viscosity is correlated with the anchoring strength, and as the viscosity increases, the anchoring strength increases. Due to this, the anchoring strength is increased by rising the temperature of the liquid crystal panel during the irradiation of ultraviolet.
Thus, the transition temperature, in which the orientation pattern is transformed from the bipolar form to the radial form, is allowed to decrease by rising the temperature of the liquid crystal panel during irradiation of ultraviolet in the process of manufacturing the liquid crystal display element, and thereby the optical hysteresis at low temperatures can be reduced to obtain the liquid crystal display element having good display performance in a wide temperature range.
It is noted that in excessively high temperatures, it takes long time for polymerization, to lead to increase in particle size of the liquid crystal droplets. In this case, if there is a scratch defect in the glass substrates, for example, due to variations in the degree of separation of the liquid crystals, variations in the particle size of the liquid crystal droplets are caused easily, so that there is a possible fear that an uniform display over the entire display screen cannot be accomplished. Due to this, it is preferable that the liquid crystal panel is irradiated with ultraviolet, with kept at a constant temperature higher than the phase-separation-generation temperature by 3-15xc2x0 C. for example.
Also, it is preferable that the intensity of ultraviolet with which the liquid crystal panel is irradiated should be rendered higher than a given intensity. This is because low intensity of ultraviolet causes decreased rate of polymerization and in turn causes increased particle size of the liquid crystal droplets, and accordingly there is a possible fear of failing to realize the uniform display. Specifically, the ultraviolet should be preferably irradiated at an intensity more than 100 mW/cm2, for example.
On the other hand, the transition temperature in which the orientation pattern is transformed from the bipolar form to the radial form can be lowered also by allowing tilt angles of the liquid crystal molecules at their interfaces with the polymer to decrease by adding an additive or equivalent. This is because, with decreasing tilt angles, the difference in energy between the bipolar-form orientation pattern and the radial-form orientation pattern increases and the bipolar-form orientation pattern is more stable in energy. Specifically, the tilt angles should be set to be 10xc2x0 or less, preferably 5xc2x0 or less to cause hard transition to the radial-form orientation pattern.
(3) 3rd inventive group
In the third inventive group, the capacitance hysteresis Chys defined by the following expression 3-1 and the optical hysteresis Thys defined by the following expression 3-2 are newly introduced as indexes to determine the optical hysteresis in association with the orientation pattern of the liquid crystal molecules, and with the aid of these indexes, the liquid crystal display element small in the optical hysteresis in the operating temperature range of the liquid crystal display element (driving temperature of the element) is accomplished. Detailed description of the capacitance hysteresis Chys will be given later.
Capacitance hysteresis Chys=(C2xe2x88x92C1)/Cmaxxe2x80x83xe2x80x83Expression 3-1,
where
C1: capacitance for any applied voltage V, in the process of rising, of a voltage-capacitance characteristic;
C2: capacitance for any applied voltage V, in the process of dropping, of the voltage-capacitance characteristic; and
Cmax: capacitance for a maximum applied voltage of the voltage-capacitance characteristic.
Optical hysteresis Thys=(T2xe2x88x92T1)/Tmaxxe2x80x83xe2x80x83Expression 3-2,
xe2x80x83where
T1: intensity of transmitted light for any applied voltage V, which is in the process of rising, of the voltage-transmittance quantity characteristic;
T2: intensity of transmitted light for the applied voltage V, which is in the process of dropping, of the voltage-transmittance quantity characteristic; and
Tmax: intensity of transmitted light for a maximum applied voltage, of the voltage-transmittance quantity characteristic.
It is noted that the term used above, xe2x80x9ca maximum applied voltagexe2x80x9d, is intended to mean an applied voltage required for the liquid crystal molecules to be fully oriented in the direction of electrical field. This voltage may be varied by variations in panel gap or the like. A general type of polymer dispersion type liquid crystal display element is so formed that a 10-15 volt is required for allowing the liquid crystal molecules to be oriented in the direction of electrical field.
The third inventive group comprises the 55th-64th aspects described below.
(55) A polymer dispersion type liquid crystal display element in which a polymer dispersion type liquid crystal is sandwiched between a pair of substrates each having an electrode at the inside thereof, said polymer dispersion type liquid crystal being such that liquid crystal droplets are dispersed and held in a continuous phase of matrix comprising polymer compound or are dispersed and held in networks of a three dimensional network form of matrix comprising polymer compound, wherein, when capacitance hysteresis in an operating temperature of said polymer dispersion type liquid crystal display element is defined by Chys=(C2xe2x88x92C1)/Cmax, the Chys for any applied voltage V is 1.5% or less, where C1 is capacitance for any applied voltage V, which is in the process of rising, of a voltage-capacitance characteristic; C2 is capacitance for the applied voltage in the process of dropping; and Cmax is capacitance for the maximum applied voltage.
(56) In the above described aspect (55), where a clear point transition temperature of said liquid crystal is let be Tni, said Chys is 1.5% or less in operating temperatures of said element falling in the range of from 5xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(57) In the above described aspect (55), where a clear point transition temperature of said liquid crystal is let be Tni, said Chys is 1.5% or less in operating temperatures of said element falling in the range of from 0xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(58) In the above described aspect (55), where a clear point transition temperature of said liquid crystal is let be Tni, said Chys is 1.5% or less in operating temperatures of said element falling in the range of from xe2x88x925xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(59) In the above described aspects (55) to (58), said maximum applied voltage is 10V or more.
(60) A polymer dispersion type liquid crystal display element in which a polymer dispersion type liquid crystal is sandwiched between a pair of substrates each having an electrode at the inside thereof, said polymer dispersion type liquid crystal being such that liquid crystal droplets are dispersed and held in a continuous phase of matrix comprising polymer compound or are dispersed and held in networks of a three dimensional network form of matrix comprising polymer compound, wherein, when optical hysteresis Thys in an operating temperature range of said polymer dispersion type liquid crystal display element is defined by Thys=(P2xe2x88x92P1)/Pmax, where P1 is intensity of transmitted light for an any applied voltage V, which is in the process of rising, of a voltage-transmittance quantity characteristic; P2 is intensity of transmitted light for the applied voltage in the process of dropping; and Pmax is intensity of transmitted light for a maximum applied voltage, and further when capacitance hysteresis Chys in an operating temperature of said polymer dispersion type liquid crystal display element is defined by Chys=(C2xe2x88x92C1)/Cmax, where C1 is capacitance for any applied voltage V, which is in the process of rising, of a voltage-capacitance characteristic; C2 is capacitance for the applied voltage in the process of dropping; and Cmax is capacitance for the maximum applied voltage, the Chys for the applied voltage with which said Thys is maximized is 0.6% or less.
(61) In the above described aspect (60), a clear point transition temperature of said liquid crystal is let be Tni, the value of said Chys is 0.6% or less in operating temperatures of said element falling in the range of from 5xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(62) In the above described aspect (60), a clear point transition temperature of said liquid crystal is let be Tni, the value of said Chys is 0.6% or less in operating temperatures of said element falling in the range of from 0xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(63) In the above described aspect (60), a clear point transition temperature of said liquid crystal is let be Tni, the value of said Chys is 0.6% or less in operating temperatures of said element falling in the range of from xe2x88x925xc2x0 C. to (Tnixe2x88x925)xc2x0C.
(64) In the above described aspects (60) to (63), said maximum applied voltage is 10V or more.
Next, the significance of the constructions described above will be described below.
In a general type of polymer dispersion type liquid crystal display element, liquid crystal molecules in the liquid crystal droplets are individually oriented in different directions while no voltage is applied to the element. Due to this, light incident on the element is scattered to produce the opaque state. On the other hand, when voltage is applied thereto, the liquid crystal molecules are oriented in the direction perpendicular to the substrates, and as a result, light can be transmitted to produce the transparent state. In addition, an intermediate state between the scattering state and the transparent state can be displayed by adjusting a level of the applied voltage. However, the polymer dispersion type liquid crystal have the optical hysteresis caused by the interfacial restrictive force, as mentioned above. This causes a difference between the transmittance in the process of voltage rise and the transmittance in the process of voltage drop even at an identical level of voltage, and resultantly causes a problem of unstable display performance in a halftone, in particular.
The inventors tried to pursue the origins of the strong optical hysteresis, particularly, of the optical hysteresis strengthened in the low temperature range, in the polymer dispersion type liquid crystal. Though there is a precedent for the measurement of the optical hysteresis (e.g. Society for information Display ""92, Pages 575-578 by S. Niiyama, et. al.), there is no precedent for the optical hysteresis measured in relation to the orientation pattern of the liquid crystals in the polymer dispersion type liquid crystal. Accordingly, the inventors measured capacitance as a physical value, which reflects the orientation pattern of liquid crystals more directly than the transmittance does, to establish the technique of estimating the orientation pattern of the liquid crystals by means of the capacitance and realized the polymer dispersion type liquid crystal display element capable to reduce the optical hysteresis in a low temperature range, based on the information obtained by the established technique.
The capacitance is a physical value originating from anisotropy in dielectric constant of the liquid crystal molecules, and so a magnitude of the capacitance reflects the orientation pattern of the liquid crystals directly. Therefore, the optical hysteresis originating from the shape and the orientation pattern of the liquid crystal droplets can be determined in relation to the behaviors of the liquid crystal molecules by gaining a knowledge of hysteresis in the capacitance (capacitance hysteresis). The optical hysteresis is a property that appears as a result of various factors, including the anchoring strength, the panel gap, and the dielectric constant, refractive index and temperature of the liquid crystal, being intricately affected each other.
The relationship between the optical hysteresis and the capacitance hysteresis is described below, based on the experimental results. Of the experimental results described below, Experiment 1 is the same as Embodiment 3-1 of the third inventive group discussed later, so the details such as measurement conditions, are shown in Embodiment 3-1.
(Experiment 1)
The measurement result on the transmittance and the capacitance hysteresis at the element temperature of 30xc2x0 C. is shown in FIG. 22 in the form of voltage-transmittance characteristics and voltage-capacitance characteristics. It is seen from FIG. 22 that a voltage corresponding to the transmittance of 10% was 4.47V and the capacitance ratio C%, corresponding to the voltage was 60%.
The capacitance ratio indicates a value defined by the following expression 3-3.
Capacitance ratio C% (C Cmax)xc2x7100xe2x80x83xe2x80x83Expression 3-3,
where Cmax is a capacitance at a maximum applied voltage and C is a capacitance in any applied voltage.
In order to confirm whether the above results can be generalized or not, various kinds of elements different in the optical hysteresis were prepared and were subjected to similar experiments. The experimental conditions are described in Example 3-1 discussed later. The experimental result is shown in FIG. 27. It was confirmed in FIG. 27 that the larger optical hysteresis the liquid crystal display element has, the smaller the capacitance ratio of a 10%-transmittance-providing voltage. In detail, a 2% or less hysteresis requires that a capacitance ratio of the 10%-transmittance-providing voltage of the voltage-transmittance characteristics be set 60% or more. Similarly, an 1% or less optical hysteresis requires that a capacitance ratio of the 10%-transmittance-proviidng voltage of the voltage-transmittance characteristics be set 66% or more.
It was proven by the above experimental results that the panel having reduced optical hysteresis can be realized by specifying the value of capacitance hysteresis.
(Experiment 2)
FIG. 23 is a plot of the results of Experiment 1, laying off the values of applied voltages as abscissa and the capacitance hysteresis Chys and the optical hysteresis Thys as ordinate, to indicate the voltage-Chys characteristics and the voltage-Thys characteristics. In this figure, a maximum of Chys is designated as Chys MAX; a maximum of Thys as Thys Max; and a value of Chys in the voltage in which the Thys reaches the maximum as Chysxc2x7Thys MAX.
It was understood from FIG. 23 that the Chys peaks at a lower voltage than the Thys and that the peak of the Chys is lower than that of the Thys.
(Experiment 3)
In order to check for the correlation between the Thys and Chys MAX, various kinds of elements were prepared under the manufacturing conditions shown in TABLE 3-1 below and were measured with respect of the voltage-capacitance characteristics with driving temperature varied. The measurement result is shown in TABLE 3-2. Shown in FIG. 24(a) is a showing of the measurement result plotted between the Thys (%) and the Chys MAX (%), and shown in FIG. 24(b) is a showing of the same plotted between the Thys (%) and the Chysxc2x7Thys MAX (%).
It was understood from FIG. 24(a) that a not more than 2% Thys requires setting the Chys MAX to be 1.5% or less, while a not more than 1% Thys requires setting the Chys MAX to be 1.0% or less.
Shown in FIG. 24(b) is a showing of the Chysxc2x7Thys MAX, the capacitance hysteresis for a voltage required for the optical hysteresis to comes to peak. It is seen from FIG. 24(b) that there is a correlation between the optical hysteresis and the magnitude of the capacitance hysteresis for the applied voltage required for the optical hysteresis to become a maximum. This correlation indicates that the element having small optical hysteresis can be produced by allowing a peak (Thys MAX) of the optical hysteresis and a peak (Chys MAX) of the capacitance hysteresis shown in FIG. 23(b) to be away from each other or by allowing the peak of the capacitance hysteresis to decrease. It can be also seen that a not more than 2% the optical hysteresis requires the capacitance hysteresis peaking at a voltage to be set to be about 0.6% or less, and a not more than 1% optical hysteresis requires the capacitance hysteresis to be set to be about 0.3% or less. The details of conditions required for preparation are omitted, though the relationship between the Thys % and the Chys MAX in a high optical hysteresis region and the relationship between the Thys% and the Chysxc2x7Thys MAX (%) are plotted in a similar manner to the above in FIG. 25(a) and FIG. 25(b), respectively. It was confirmed in these FIGS. also that there exists the above-described correlation therebetween.
It is noted that the difference between the peak of the optical hysteresis (Thys MAX) and the peak of the capacitance hysteresis (Chys MAX) is due to the following. The magnitude of capacitance is a physical value which directly reflects the degree of rise of liquid crystal molecules (the angle between the major axis of rising molecule and the substrate), whereas the transmittance is not in a one-to-one correspondence with the angle above. For example, even when the liquid crystal molecules is raised slightly by application of voltage, the scattering of light is still maintained.
To secure the practical display performance requires the optical hysteresis of the element to be set to be preferably 2% or less, more preferably 1% or less. The reason therefor is that the element of a not more than 2% optical hysteresis is usable as display elements, such as data projections, mainly for displaying characters or letters, and further the element of a not more than 1% optical hysteresis ensures sufficient gray scale display performance so that it can be used to displays for displaying images or equivalent.
As described above, the polymer dispersion type liquid crystal display element having small optical hysteresis can be surely produced by specifying the capacitance hysteresis. In general, the magnitude of the optical hysteresis varies depending on the temperature of element, so it is essential that the optical hysteresis is reduced in the temperature range in which the liquid crystal display element is worked (the driving temperature range of the element). Specifically, the element having a not more than 2% optical hysteresis in the temperature range of 10xc2x0 C. to 80xc2x0 C. is usable to projection type displays, and the element having the optical hysteresis of not more than 2% in the temperature range of xe2x88x9220xc2x0 C. to 80xc2x0 C. is usable as displays on board of automobiles or equivalent.
The optical hysteresis can be estimated by the degree of difference between the peaks, rather than by a value of the capacitance hysteresis at a peak of the optical hysteresis.
(4) 4th inventive group
According to this inventive group, attention is given to the relationship between a surface tension of a droplet-formed liquid crystal of a liquid crystal optical element and a critical surface tension of an insulating film or a polymer compound in the form of a matrix of liquid crystal droplets. The invention as this inventive group aims to improve temperature dependency, a response-to-electric field characteristic, and the like, of the liquid crystal optical element by holding this relation within a given range.
Further, the invention aims to achieve the object by improving the material of the insulating film.
Furthermore, the invention aims to achieve the same object by selecting the material of the polymer compound and to simultaneously provide the method for producing an excellent liquid crystal optical element with efficiency.
The fourth inventive group comprises the 65th-18th aspects.
The 65th aspect is characterized in that in a polymer dispersion type liquid crystal display element in which a polymer liquid crystal complex in which droplets of liquid crystal are dispersed in a polymer compound is filled in a space between a pair of electrodes supported by substrates and covered with insulating films, surface tension xcex3LC of said liquid crystal and critical surface tension xcex3P of said insulating films meet the relation of Expression 4-1:
xcex3LCxe2x88x92xcex3P less than 0xe2x80x83xe2x80x83Expression 4-1.
According to the 66th aspect, in the above described aspect 65, said surface tension xe2x80x2YLC of said liquid crystal and said critical surface tension xe2x80x2YP of said insulating films further meet the relation of Expression 4-2:
xe2x88x921xc2x7dyne/cm less than xcex3LCxe2x88x92xcex3Pxe2x80x83xe2x80x83Expression 4-2.
The 67th aspect is characterized in that in the above described aspect 66, said polymer.liquid crystal complex has a value of xcex3, which indicates steepness of a threshold of a scattering-transmittance characteristic, falling in the range of 1.95 to 2.25.
The 68 aspect is characterized in that in a polymer dispersion type liquid crystal display element in which a polymer liquid crystal complex in which droplets of liquid crystal are dispersed in a polymer compound is filled in a space between a pair of electrodes supported by substrates and covered with insulating films, surface tension xcex3LC of said liquid crystal and critical surface tension xcex3P of said insulating films meet the relation of Expression 4-3:
0 less than xcex3LCxe2x88x92xcex3P less than 1xc2x7dyne/cmxe2x80x83xe2x80x83Expression 4-3.
The 69th aspect is characterized in that in the above described aspect 65, 66, 67 or 68, said insulating films are made of polyamino acids, polyamino acid derivatives or proteins.
The 70th aspect is characterized in that in a polymer dispersion type liquid crystal display element in which a polymer liquid crystal complex in which droplets of liquid crystal are dispersed in polymer compound is filled in a space between a pair of electrodes supported by substrates and covered with insulating films, critical surface tension xe2x80x2YP of said polymer compound and surface tension xcex3LC of said liquid crystal meet the relation of Expression 4-4:
xcex3P greater than xcex3LCxe2x80x83xe2x80x83Expression 4-4.
The 71st aspect is characterized in that in the above described aspect 70, said polymer compound is formed by polymerization of polymerizable monomer and polymerizable oligomer, and further at least one of said polymerizable oligomer and said polymerizable monomer has a polar group.
The 72nd aspect is characterized in that in the above described aspect 71, said polar group is at least one group selected from the group consisting of a hydroxyl group, a carboxyl group and an imino group.
The 73rd aspect is characterized in that in the above described aspect 70, 71 or 72, said xcex3P and said xcex3LC meet the relation of an expression 4-4 in a full temperature range (xe2x88x9210xc2x0 C. to 60xc2x0 C.) in actual operation of the display element:
xcex3P greater than xcex3LCxe2x80x83xe2x80x83Expression 4-4.
The 74th aspect is characterized in a method of producing a polymer dispersion type liquid crystal display element comprising:
the filling step in which a polymer precursor.liquid crystal mixture, including a liquid crystal and a polymer precursor, from which a polymer compound which allows the relation of Expression 4-4 to hold between said liquid crystal and said polymer compound is formed by polymerization, is filled in a space between a pair of electrodes supported by substrates and covered with insulating films, and
the polymer.liquid crystal complex forming step in which said polymer compound in said polymer precursor.liquid crystal mixture, after filled, is polymerized to form said polymer compound which allows the relation of Expression 4-2 to hold, while also polymer.liquid crystal complex in which droplets of said liquid crystal are dispersed in the formed polymer compound is formed:
xcex3P greater than xcex3LCxe2x80x83xe2x80x83Expression 4-4,
where xcex3P is critical surface tension of the polymer compound and xcex3LC is surface tension of the liquid crystal.
The 75th aspect is characterized in that in the above described aspect 74, said polymer precursor to be treated in said filling step and said polymer precursor.liquid crystal complex forming step is composed of polymerizable monomer and polymerizable oligomer, and further at least one of the polymerizable oligomer and the polymerizable monomer has a polar group.
The 76th aspect is characterized in that in the above described aspect 75, said polar group is at least one group selected from the group consisting of a hydroxyl group, a carboxyl group and an imino group.
The 77th aspect is characterized in that in the above described aspect 74, 75 or 76, said polymerization in said polymer.liquid crystal complex forming step is produced by the method of said polymer precursor liquid crystal mixture placed between said pair of substrates being irradiated with ultraviolet.
Other aspects are characterized by any proper combination of the 65th to 77th aspects described above.
The significance of the above described constructions will be described below.
Significance of the 65th to 69th aspects
Of electro-optical characteristics of the general type liquid crystal display elements, the most essential characteristic is a scattering-transmittance characteristic indicating the relationship between the transmittance of light to vertical incident light and an applied voltage. A lot of experiments the inventors made showed that values of xcex3, which indicate steepness of the threshold characteristic of the scattering-transmittance characteristic, were related to the temperature dependency and the response time of the polymer dispersion type liquid crystal display element, and the inventors found out a certain relative criterion that the values of xcex3, at which optimal temperature dependency and response time can be obtained, range from about 1.7 to about 2.3. It is noted that the value of xcex3 used here is the one defined by xcex3=V90/V10, where V10 (volt) is a voltage required for the transmittance of light of the liquid crystal display element to vary by 10% and V90 (volt) is a voltage required for the transmittance of light of the liquid crystal display element to vary by 90%, when the maximum transmittance of light of the liquid crystal display element is set to be 100%.
Further, it was also found out that the value of xcex3, which indicates the steepness of the threshold characteristic of the liquid crystal display element, is related to a critical surface tension xcex3p of an insulating paint film material and a surface tension xcex3LC of a liquid crystal to be formed into liquid crystal droplets in the polymer dispersion type liquid crystal, so that the value of xe2x80x2Y can be controlled by adjusting the mutual surface tensions. The term of xe2x80x9csurface tensionxe2x80x9d used herein is intended to mean surface energy. The significance is specifically described below.
FIG. 28 is a sectional view of a main structure, illustrated in a simplified manner, of the liquid crystal display element of the present invention. The liquid crystal display element of the invention is not essentially different from the conventional type one in the mechanical structure itself.
The polymer dispersion type liquid crystal display element in the 65th to 69th aspects comprises, as shown in FIG. 28, a pair of opposing, support substrates 411, 412 made of transparent glasses or crystals and having inner surfaces on which transference electrodes 413 of indium tin oxide and insulating paint films 414 made of various kinds of insulating paint film materials described later and covering the transference electrodes 413 are laminated; and polymer dispersion type liquid crystal 417 which is filled in between the transference electrodes 413 oppositely disposed with the insulating paint films 414 confronting each other and in which droplets 416 of liquid crystal are dispersed in polymer compound 415. The surface tension xcex3LC of the liquid crystal and the critical surface tension xcex3p of the insulating paint films 414 then meet any one of the requirements of xcex3LCxe2x88x92xcex3p less than 0 (hereinafter it is called conditional expression 1), xe2x88x92dyne/cm less than xcex3LCxe2x88x92xcex3p less than 1 dyne/cm (hereinafter it is called conditional expression 2) or xe2x88x921 dyne/cm less than xcex3LCxe2x88x92xcex3p less than 0 (hereinafter it is called conditional expression 3). The peripheries of the element, not shown, are joined together by sealing members produced by curing acid anhydride curing epoxy resin reinforced by glass fiber, for example, so as to form a closed container formed in one piece as a whole.
The value of xcex3, which indicates steepness of threshold characteristic of the scattering-transmittance characteristic which is a basic electro-optical characteristic of the liquid crystal display element, is controlled in association with adjustment of the relation between the critical surface tension xcex3p of the insulating paint films 414 covering the transference electrodes 413 on the interior surfaces of the support substrates 411, 412 and contacting with the polymer dispersion type liquid crystal 417 and the surface tension xcex3LC of the liquid crystal to be formed into droplets 416 of liquid crystal in the polymer dispersion type liquid crystal 417. It is thought that this is caused by the following operation.
The provision of the insulating paint films 414 contacting with the polymer dispersion type liquid crystal 417, and also the interval between the pair of insulating paint films being as narrow as about 13 xcexcm, contribute to cause an interactive force, such as Van der Waals force or polarity-polarity interactive force, to work between the insulating paint films and the liquid crystals. The interactive force further exerts on the droplets 416 of the liquid crystal material in the interior of the polymer dispersion type liquid crystal 417 apart from the surfaces of the insulating paint films 414.
The existence of this interactive force allows the interfacial restrictive force acting on the liquid crystal droplets 416 from the polymer matrix 415 (equivalent of a torque required for the transition of orientation) to vary, which in turn allows the value of xcex3 which indicates steepness of a threshold level in the liquid crystal display element to vary. Then, when the value of xcex3 of the liquid crystal display element vary, the temperature dependency of the interfacial restrictive force acting on the liquid crystal droplets 416 from the polymer compound 415 varies with reference to the interactive force applied from the insulating paint films 1. As a result of this, not only the temperature dependency of the driving voltage, etc. but also the response time become optimal. It seems that under the circumstance under which the liquid crystal material is more prone to got wet by the insulating paint films 414 due to the both being polarized, for example, the interactive force acting on the liquid crystal material from the insulating paint films 414 increases in strength, and under the circumstance, the temperature dependency is also more prone to improvement.
Significance of the 70th to 77th aspects
As for the relationship between the critical surface tension of the polymer compound and the surface tension of the liquid crystal material, when the critical surface tension xcex3P of the polymer compound and the surface tension xcex3LC of the liquid crystal meet the requirement of xcex3P greater than xcex3LC, the interface polymer/liquid crystal becomes stable in energy and then the liquid crystal molecules (director) are oriented at small angles by the polymer compound interfacial tension to produce the bipolar-form orientation shown in FIG. 41(1). The bipolar-form orientation pattern can allow the orientations of the liquid crystal molecules to be changed by less kinetic energy than the radial-form orientation pattern, so that the response to electric field (response time) through the on/off of voltage is improved and also the hysteresis is reduced.
Although the liquid crystal is easily affected electro-optically by variations in temperature, since the xcex3P greater than xcex3LC provides surface energy of the polymer compound larger than the surface energy of the liquid crystal material, the degree of the liquid crystal being electro-optically effected by the variations in temperature is relatively decreased. Thus, the temperature dependency of the voltage optical characteristics can be reduced.
As for the manufacturing method, it is preferable for the control of diameter and dispersion of liquid crystal particles that the mixture of polymer precursor and liquid crystal material is irradiated with ultraviolet in the phase separation method, whereby the polymer precursor is polymerized and phase-separated. The significance is specifically described below.
In the 70th to 77th aspects, various kinds of liquid crystal materials, such as nematic liquid crystal, cholesteric liquid crystal, and smectic liquid crystal, which exhibit a liquid crystal state in around room temperature, can be used as the liquid crystal material. The liquid crystal material may be used singularly or in combination of two or more kinds. Also, the liquid crystal material may be used with containing a two-tone coloring matter therein. For example, polymer.liquid crystal complexes in which two-tone coloring matters having different colors are contained may be laminated to form an optical element capable of full-color display.
The polymer compound used in the embodied forms has light permeability and holds the expression 4 in association with the liquid crystal forming the dispersion phase. The polymer.liquid crystal complexes capable to hold the expression 4 enable the liquid crystal molecules to be oriented at small tilt angles to the wall of matrix, so that the response-to-electric-field of the liquid crystal display element and the temperature dependency of the voltage optical characteristics is reduced. Preferably, the requirement of the expression 4 should be always allowed to hold in a full temperature range (xe2x88x9210xc2x0 C. to 60xc2x0 C.) in actual operation of the liquid crystal display element. This formation can provide a good and stable display in a wide temperature range. The significance of the requirement of the expression 4-4 is discussed later.
xcex3P greater than xcex3LCxe2x80x83xe2x80x83Expression 4-4
where xcex3LC is surface tension of liquid crystal, and xcex3P is critical surface tension of polymer compound.
Further, it is preferable that the polymer compound used has a good affinity for the liquid crystal; For example, the polymer compounds having a polar group such as a hydroxyl group, a carboxyl group or an imino group should be preferably used. In addition, in consideration of the manufacturing circumstances, it is desirable that the polymer compound be produced by the polymer precursor being injected in between the electrodes and thereafter polymerized. In this step, it is preferable to use means for allowing polymerizable monomer and polymerizable oligomer to be polymerized to produce the polymer compound, in terms of productivity of good quality of polymer.liquid crystal complex.
Further, in the case of the polymer compound being produced by the polymerization after injection, it is preferable that the polymerizable monomer and polymerizable oligomer are both used and at least one of the monomer and the oligomer has the polar group, more preferably, the polar group should include hydroxyl group, carboxyl group or imino group. This is because, when at least one of the monomer and the oligomer has the polar group including the hydroxyl group, the polymer compound having high hydrophilic nature (affinity) can be produced, and the polymer compound having high hydrophilic nature thus produced can provide better wetting in the interface polymer/liquid crystal to improve the response to electric field and the voltage optical characteristics of the element. To be more specific, energetic stability in the interface polymer/liquid crystal is improved and the liquid crystal molecules can be allowed to exist at smaller angles to a wall surface of the polymer. In addition, the better wetting reduces the tendency of transition of orientation pattern of the liquid crystal molecules (transition from the bipolar form to the radial form) due to variations in temperature, which in turn can allow the temperature dependency of the liquid crystal display element to be reduced.
The polymerizable polymer precursors which may be used include various kinds of polymerizable materials which are polymerized by light (ultraviolet) and heat to produce a transparent polymer compound. In general, the monomer or oligomer having a polymerizable functional group such as acrylate, methacrylate and epoxy is used. To be more specific, the polymerizable monomers which may be used include 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, monohydroxyethyl acrylate phathalate, neopentyl glycol diacrylate and hexanedioldiacrylate. The oligomers which may be used include urethan acrylate, 1,6 hexanedioldiacrylate, pentaerythlytoldiacrylate monostearate, oligourethane acrylate, polyester acrylate and glycerine diglycidylether.
Examples of the polymerizable polymer precursors having the hydroxyl group are M-5700 (monomer) and M233 (oligomer) available from TOAGOSEI CO., LTD., and an example of the polymerizable polymer precursor having the carboxyl group is M-5400 (monomer) available from TOAGOSEI CO., LTD. Further, examples of the polymerizable polymer precursors having the imino group are M-1200 (oligomer) and M-1600 (oligomer) available from TOAGOSEI CO., LTD. and UF-8001 available from KYOEISHA CO., LTD.
For meeting the requirement of the xcex3P greater than xcex3LC in a wide actual operating temperature range, the polymer material and the liquid crystal material must be combined properly. The polymer compound which is polymerized by use of the polymerizable monomer and/or polymerizable oligomer having the hydroxyl group, carboxyl group and imino group, as described above, combined with the liquid crystal material compatible therewith, e.g. MT5524 available from CHISSO PETROCHEMICAL CORPORATION can realize the liquid crystal display element capable of meeting the requirement of the xcex3P greater than xcex3LC in a wide actual operating temperature range.
The polymer.liquid crystal complex, which is a main component of the polymer dispersion type liquid crystal display element according to the embodied forms, can be produced in any known manner, using the above-listed materials. To be more specific, the known manners which may be used includes a casting process in which a liquid crystal material and a polymer material, after dissolved in a common solvent, are cast; an emulsion process in which the liquid crystal, after emulsified in aqueous solution of water-soluble polymer, is cast; and a phase separation process in which an uniform solution of liquid crystal and polymer forming material is prepared and then is phase-separated by polymerization.
Of the above-described processes, the phase separation process is desirable for the liquid crystal polymer dispersion type liquid crystal display element of the above described embodied forms. More preferable one is a photopolymerization phase separation process (using ultraviolet) using the molymerizable monomer and polymerizable oligomer mentioned above. That is because, in the photopolymerization phase separation process, the liquid crystal is fully dispersed in advance in the polymer precursor having a low viscosity and thereafter the polymer precursor is polymerized to cause the phase separation, so that the particle size of and the condition of dispersion of the liquid crystal droplets can be easily controlled to produce a desirable polymer.liquid crystal complex.
In the case of the photopolymerization phase separation process, a polymerization initiator should preferably be added to the polymer precursor, for smooth polymerization of the polymerizable polymer precursor. The polymerization initiators which may be used include commercially available polymerization initiators, such as Darocure 1173, Darocure 4265 and Irgacure 184 available from CIBA-GEIGY LTD., in addition to Benzyl Methyl Ketal. Two or more kinds of these may be used in combination.
The polymerization of the polymer precursor may be performed in such a manner that the polymer precursor.liquid crystal mixture, which may include the polymerization initiator, is placed in between a pair of substrates and thereafter is irradiated with ultraviolet from the top of the substrates. In this case, the intensity of ultraviolet to be irradiated should be 8 mW/cm2 or more, preferably 150 mW/cm2 or more, and further preferably 200 mW/cm2 or more. The intensity of ultraviolet of 150 mW/cm2 or more allows the optical hysteresis to decrease when the operating temperature of the liquid crystal display element becomes high temperatures, and the intensity of ultraviolet of 200 mW/cm2 or more advantageously allows the orientation transition temperature to decrease significantly.
On the other hand, it is also preferable that the intensity of ultraviolet may be reduced to 30 mW/cm2 or less, to attempt to reduce the driving voltage of the liquid crystal display element. With the intensity of ultraviolet of 30 mW/cm2 or less, the polymer compound is produced so slowly that the particle size of the liquid crystal droplets can be increased. With the liquid crystal droplets having large particle size, the interfacial restrictive force of the polymer compound to the liquid crystal droplets is relatively reduced so that the element capable to be driven through an application of a reduced voltage can be obtained.
Further, the particle size of the liquid crystal droplets of the liquid crystal display element should be set 0.8 xcexcm to 2.5 xcexcm, preferably, 1 xcexcm to 2 xcexcm, depending on what equipment is used and what is the intended use. That is because the liquid crystal droplets of 0.8 xcexcm to 2.5 xcexcm provides the sufficient scattering effect, and the liquid crystal droplets of 1 xcexcm to 2 xcexcm enables the liquid crystal panel to be driven through an application of a low voltage available for the TFT drive.
It is preferable that the cell gap for the polymer.liquid crystal complex to be filled in should be 5 xcexcm or more, preferably 10 xcexcm to 15 xcexcm. This set cell gap can allow both of improved light scattering properties and reduced driving voltage to be achieved by setting the particle size of the liquid crystal droplets properly.
It is to be noted that the most characteristic feature of the polymer dispersion type liquid crystal display element of the above described aspects of the invention is in that the xcex3P greater than xcex3LC holds, and no particular limitation is given to any other factors than the factor of the xcex3P greater than xcex3LC. Thus, the liquid crystal display element of the invention can be produced by any known producing methods plus the requirement of the xcex3P greater than xcex3LC. This enables the polymer dispersion type liquid crystal display element having the features of the embodied forms of the invention to be produced with relative ease.
Incidentally, with the polymer dispersion type liquid crystal in which microscopic liquid crystal droplets are dispersed in a matrix phase of polymer compound, contact areas of the liquid crystals with the polymer compound are significantly large, so that the orientation pattern of the liquid crystal molecules is greatly affected by the interfacial restrictive force (physico-chemical force) of the polymer compound. Accordingly, the polymer dispersion type liquid crystal display element is poorer in response to electric field, as compared with a conventional type liquid crystal display element which is regulated by the substrates only. In addition, the intensity of the interfacial restrictive force is susceptible to temperature, so that, when operating temperature of the element varies, the response to electric field (response time, in particular) and voltage optical characteristics (transmission for an applied voltage) of the liquid crystal molecules vary. Accordingly, the polymer dispersion type liquid crystal display element has a disadvantage of lacking stability in display performance, as compared with conventional type elements such as the TN mode of liquid crystal display element.
There is literature on the interfacial restrictive force of the polymer compound e.g. Sov. Phys. JETP 58(6), 1983, Liquid Crystal Dispersions written by P. S. Drzaic, World Scientific 1996, according to which the bipolar-form orientation having two poles is produced under high temperatures of about room temperatures or more, as shown in FIG. 41(1), and the radial-form orientation of liquid crystals being oriented vertically to the interfaces is produced under low temperatures, as shown in FIG. 41(2).
FIG. 41(1) shows the state of liquid crystal molecules being oriented toward two poles along a spherical surface, and FIG. 41(2) shows the state of liquid crystal molecules being oriented with one ends thereof orienting toward the spherical surface and the other ends orienting toward the center of the sphere.
The relationship between the interfacial restrictive force of the polymer compound and the orientation of the liquid crystal molecules is considered below, based on the above literature. It is thought that the liquid crystal molecules in the liquid crystal droplets surrounded by the polymer compound are strongly affected by the physico-chemical force from the polymer compound of matrix and thereby are so oriented that free energy of the interface polymer/liquid crystal can be minimized. As a result, the state of the liquid crystal molecules being oriented in parallel to the interfaces, i.e., the bipolar-form orientation shown in FIG. 41(1), is higher in energetic stability in the polymer/liquid crystal interfacial boundary than the radial-form orientation of the liquid crystal molecules being oriented vertically to the interfaces as shown in FIG. 41(2). From this point, the liquid crystal molecules of the polymer/liquid crystal complex preferably take the bipolar-form orientation, rather than the radial-form orientation which is more susceptible to temperature, while no voltage is applied.
On the other hand, it is desirable for obtaining a satisfactory contrast ratio to allow the liquid crystal molecules to be oriented in parallel to the substrates when no voltage is applied and allow the same to be oriented vertically to the substrates quickly when a voltage larger than a threshold voltage is applied. In the radial-form orientation, however, a part of liquid crystal molecules are inevitably oriented vertically to the substrates when no voltage is applied as well. Therefore, it is hard to obtain a satisfactory contrast ratio. In addition, since the liquid crystal molecules are oriented with higher energy, it is difficult to allow them to be quickly oriented vertically to the substrates. From this point also, the radial-form orientation is not desirable.
Hereupon, in the above described embodied forms, the element is formed by the polymer compound material and the liquid crystal material being selected so suitably that the xcex3P greater than xcex3LC can hold between the critical surface tension xcex3P of the polymer compound and the surface tension xcex3LC of the liquid crystal material. In this form of the element, the orientation pattern of the liquid crystal molecules during no voltage being applied takes the bipolar form and the liquid crystal molecules are oriented at small tilt angles to the wall surface of the polymer compound. This can provide improved response to electric field and reduced temperature dependency of the voltage optical characteristics.
Further, when the requirement of the xcex3P greater than xcex3LC is met by using polymer compound having polar group, such as hydroxyl group, carboxyl group or imino group, good wettability (affinity) between the liquid crystal droplets and the polymer compound surrounding them is obtained, so that the liquid crystal molecules are allowed to stably exist in the interface polymer/liquid crystal. This enables the tilt angles of the liquid crystal droplets to the wall surface of the polymer compound to be further reduced, to make it hard to cause the orientation transition between the bipolar form and the radial form. This can provide the result of producing the liquid crystal display element having improved response to electric field and reduced temperature dependency of the voltage optical characteristics.
It is noted that the polymer dispersion type liquid crystal referred to in the fourth inventive group is not limited to the polymer.liquid crystal complex only wherein droplets of the liquid crystal are interspersed in an island form in the polymer compound, but may include not only the one wherein the droplets of the liquid crystal are partially associated in series with neighboring droplets but also the one (polymer network liquid crystal) wherein the droplets of the liquid crystal are held in networks of the polymer compound of a three dimensional network. However, the formation for the droplets of the liquid crystal to be held in the networks of the polymer compound of the three dimensional network form does not take the bipolar-form orientation pattern in general, because the interfacial restrictive force does not act on the droplets of the liquid crystal uniformly. It is known however that even this formation allows the liquid crystal molecules to be oriented vertically under low temperatures, to take the radial-like form orientation, so there may arise the above-described problem of strong temperature dependency in the response to electric field. Accordingly, even in this case, the feature of xcex3P greater than xcex3LC of the present invention works effectively.
Finally, the principle of the liquid crystal display element of the fourth inventive group is illustrated in FIG. 40. Shown in this figure is the state of incident light 422 being turning transmitted light 424 and scattered light 423 due to different molecular orientations in the liquid crystal droplets.